Alloys resistant to high temperatures



United States Patent Ofitice 3,172,759 Patented Mar. 9, 1965 3,172,759 ALLGYS RESHSTANT T HEGH TEMPERATURES William T. Kaarlela, Fort Worth, Tex, assignor to General Bynarnics Corporation, San Diego, Qalifi, a corporation of Delaware No Drawing. Filed Sept. 13, 1961, Ser. No. 137,724 (Claims. (Cl. 75-l34) This invention relates in general to alloys capable of resisting high temperatures and more particularly, to alloys of such character well suited for the brazing of columbium and columbium base alloys and having additional utility as high tern erature protective coatings and as casting alloys.

It has become increasingly important, particularly in aircraft applications, to use materials which are capable of withstanding extremely high temperatures. One such material is columbium, which is a refractory metal pos sessing a melting point in the vicinity of 4400 F. From a design standpoint, it is an excellent material because of its high strength to weight ratio in the 2000 to 2500 F. range of service temperatures. It does not strainharden rapidly, allowing cold working up to 99% without annealing, and it is therefore particularly suitable for the forming of parts of complex shape. columbium is also characterized by moderate density (comparable with iron and nickel), and a high melting point, with good strength retention above the useful range of currently available alloys.

Useful though it is in high temperature areas, joinder of the metal and its alloys presents problems. Thus, although it may be Welded, nitrogen contamination is an obstacle since it causes an increase in the tendency for crater cracking, serious loss of ductility and an increase in the transition temperature. Welding also presents the problem or" loss of strength due to recrystallization. in the handling of high strength columbium alloys, recrystallization occurs between 2200 and 2800 F, depending upon the alloy makeup. Welding involves these high temperatures and where recrystallization as a result occurs, a loss of approximately 50% in tensile strength may be anticipated.

However, it has been found that by using the alloys of this invention, excellent joinder of the metals is efected and a high temperature resistant joint is produced which is compatible with the strength characteristics of the joined materials. Argon gas is utilized to prevent oxidation of the columbium and its alloys during the brazing, only moderate efforts being necessary for purification of the gas preparatory to brazing.

The alloys of this invention are additionally useful as coatings for columbium and for other materials characterized by low oxidation resistance at elevated temperatures, i.e., of approximately 2000 F.

Further utility for these alloys is found in casting applications and in other areas of metal forming where adequate oxidation resistance at elevated temperatures is difficult to achieve and maintain and where both structural and non-structural provisions are a requirement.

Accordingly, it is an object of this invention to provide alloys well suited for the brazing of columbium and columbium base alloys, which alloys are capable of providing excellent joint strength at elevated temperatures.

It is another object of this invention to provide alloys of the character described which do not require excessively h gh temperatures for brazing and which possess adequate ductility.

A it rther object is to provide alloys, as described, which do not cause excessive erosion of columbium and columbium base alloys when applied thereto in brazing applications.

Yet a further object is to provide alloys suitable as protective coatings for preventing oxidation of materials having a high susceptibility thereto at elevated temperatures.

Another object is the provision of alloys adapted to casting applications which call for materials possessing adequate oxidation resistance at elevated temperatures.

These and other objects and advantages of this invention will become apparent from the following description of the alloys and their characteristics and the claims directed thereto.

In general the alloys of this invention include as matrix elements chromium and palladium in the percentage-by- Weight ranges indicated. Each of these matrix elements is characterized by its compatibility with columbium and the alloys thereof. Germanium is used as an additional alloying element and is proportioned as set forth below. The resulting alloys have produced brazed joints possessing excellent joint strength at high temperatures. This brazing has been effected without the use of extremely high temperatures. Ability of the alloys to withstand high temperatures with a minimumof deterioration has contributed to their further utilization as coatings, protecting against oxidation of materials susceptible thereto. Formability at practicable temperatures and the aforementioned excellent high temperature characteristics further dictate use of the alloys in the area of metal fonning, particularly in casting applications. It is to be understood that percentages used herein in the specification and claims to describe ingredient proportions are percentages by weight unless stated to be otherwise.

Chromium, serving as one of the matrix constituents of the alloys, has been found to be quite compatible with columbium, forming a diffusion layer which shows interaction at the interface with the brazing alloy. As used herein, the general range of chromium is from about 35 to about 74% by weight of the alloy. A preferred range has been found to be from about 38 to about 59%. Superior alloy compositions have been established incorporating the specific proportions of chromium tabulated below.

Palladium, like chromium, serves as a matrix element and has similarly been found to be quite compatible with columbium, forming a narrow alloy layer. It enters into the alloys of this invention in the general range of from about 25 to about 65%. Its preferred range is from about 30 to about 62%. Superior alloy compositions include palladium in the quantities set forth below.

Germanium is included as a major alloy element being present in the alloy .in the general range of from about 1 to about 26%. The preferred range for germanium is from about 15 to about 20%. Excellent results have been achieved with alloys incorporating germanium as indicated. Germanium shows no obvious harmful effects upon the alloy, its effects if any in this respect, being masked by other alloy elements.

Although the process for alloy formulation is subject to variation, the alloys of this invention have for test purposes been formulated by mixing the elemental ingredients together in the desired proportions in powder form. The mixture is subsequently briquetted into a compact and are melted in a cold hearth copper crucible. If ductile, the alloy is then rolled to form a foil, or in the alternative, broken into a powder to be used in such form.

As formulated herein, the alloys of this invention have taken the powder form. Application is effected by mixing the powder alloy with polyvinyl alcohol in a slurry, which is then painted on the joint to be brazed. The alloy is then heated to a temperature above its melting point, using argon gas for protection against oxidation.

As has been earlier mentioned, moderate efforts should be made to purify the argon gas. Herein this has been accomplished satisfactorily by passing the argon through a glass-activated alumina dryer, a 100 F. Dry Iceacctone cold trap, and a closed zirconia tube filled with titanium strips and operating at 1750 F. The protection offered by the argon gas is important. Should atmospheric contamination occur it Will be reflected in reduced flow and wetting of the brazing alloy.

For purposes of the tests the results of which are reflected in the tabulations below, brazed lap shear test specimens were made up using for the members to be joined a columbium alloy incorporating by Weight titanium and 10% molybdenum. Time at temperature prior to testing was 1 minute. Using an A-frame type of lap-shear tension test setup, failure was made to occur Within 1 minute by steadily increasing the mechanical stress upon the specimen by means of a floating screw. Stress was measured by means of a load link in conjunction With a strain recorder. Specimens were confined under a protective argon atmosphere during heating, testing, and cooling.

Each of the alloys of this invention is set out in the table below with an indication both as to the general range of its ingredients and as to the specific composi tion of the particular alloy or alloys tested. Also shown are the alloy melting temperatures and the results of shear tests conducted at elevated temperatures with regard to each brazed joint.

It is the above qualities of the alloys of this invention which adapt them for use in metal forming application. Quite obviously superior corrosion resistance is ofiered at elevated temperatures and with melting points as indicated, forming is not a problem. The alloys can be made up in the manner indicated above or in other conventional fashion and then formed.

Numerous high temperature oxidation studies of the above alloys have been conducted. Visual and microstructural studies indicate the usefulness of these alloys as protective coatings for materials having low resistance to oxidation at elevated temperatures. Application or" the alloys may be accomplished by painting or spraying the alloy in powdered form on the surface of the metal to be protected and then heating the coated assembly until the powdered alloy melts to form a continuous surface coating. The alloys designated number 2 in the table above each showed percent Weight changes of less than 5% after 100 hours in 2000 F. air, indicating their quality as protective coatings.

What I claim is:

1. An alloy characterized by its ability to Withstand high temperatures and by weight consisting of from about 35 to about 74% chromium, from about to about 65% palladium, and from about 1 to about 20% germanium.

2. An alloy characterized by its ability to Withstand high temperatures and by Weight consisting of about 50% chromium, from about to about palladium, and from about 15 to about 20% germanium.

3. An alloy characterized by its ability to withstand high temperatures and by Weight consisting of from about to about 74% chromium, from about 25 to about 40% palladium, and from about 1 to about 20% germanium.

4. An alloy characterized by its ability to Withstand high temperatures and by Weight consisting of about chromium, about 30% palladium, and about 20% germanium.

5. An alloy characterized by its ability to Withstand high temperatures and by Weight consisting of about 50% chromium, about 35% palladium, and about 15% germanium.

References Gated by the Examiner UNITED STATES PATENTS 1,471,326 10/23 Copland 75-1'72 FOREIGN PATENTS 373,725 4/23 Germany.

714,820 12/41 Germany.

OTHER REFERENCES Raub et al.: Die Palladium-Chrom-Legicrunger, Z. Metallkunde, vol. 45, 1954, pages 648-650.

DAVID L. RECK, Primary Examiner.

RAY K. WINDHAM, ROGER L. CAMPBELL,

Examiners. 

1. AN ALLOY CHARACTERIZED BY ITS ABILTIY TO WITHSTAND HIGH TEMPERATURE AND BY WEIGHT CONSISTING OF FROM ABOUT 35 TO ABOUT 74% CHROMIUM, FROM ABOUT 25 TO ABOUT 65% PALLADIUM, AND FROM ABOUT 1 TO ABOUT 20% GERMANIUM. 